1. Field of the Invention
The present invention relates to elastic-wave filters used for, for example, band-pass filters of cellular phones, and in particular, relates to elastic-wave filters including ladder circuit structures.
2. Description of the Related Art
To date, surface-acoustic-wave filters have been widely used as band-pass filters of cellular phones for RF stages. Japanese Unexamined Patent Application Publication No. 2000-174586, for example, discloses a surface-acoustic-wave ladder filter as an example of such band-pass filters. In such a surface-acoustic-wave ladder filter, the resonant frequency frs of series arm resonators and the antiresonant frequency fap of parallel arm resonators are set adjacent to the central frequency in a passband. On the other hand, the antiresonant frequency fas of the series arm resonators is set at an attenuation pole adjacent to a high frequency region of the passband, and the resonant frequency frp of the parallel arm resonators is set at an attenuation pole adjacent to a low frequency region of the passband. This forms the passband.
As described in paragraph [0297] in Japanese Unexamined Patent Application Publication No. 2000-174586, the aperture, i.e., the overlap width of electrode fingers of an IDT electrode of the series arm resonators is reduced and the number of pairs of electrode fingers of the series arm resonators is increased. With this, electrical resistance is reduced, and as a result, loss can be reduced. Moreover, as described in paragraph [0300] in Japanese Unexamined Patent Application Publication No. 2000-174586, loss caused by diffraction of surface acoustic waves is increased when the overlap width is extremely reduced. However, the diffraction loss more greatly affects the antiresonance characteristics than the resonance characteristics. Therefore, the diffraction loss does not cause serious problems to the series arm resonators having a resonant frequency frs located adjacent to the central frequency of the passband and an antiresonant frequency fas located outside the passband since the effect thereof is small. Therefore, the overlap width is usually reduced as much as possible.
Recently, extension of passbands, increase in attenuation, and reduction in loss have been strongly demanded for band-pass filters of cellular phones. To this end, the resonant frequencies of a plurality of series arm resonators are made different from each other so that the passbands and the attenuation are increased.
When the resonant frequencies of the series arm resonators differ from each other, however, the loss within the passbands cannot be sufficiently reduced. In particular, the loss in a high frequency region within the passband is disadvantageously increased.